When implants are splinted together it adds many benefits.
In my practice I always splint crowns together and have done so for the past 15 years. I also make sure when I do this that the splinted crowns are able to be cleaned by having accessible areas between the crowns. The benefits of splinting are numerous.

I agree that splinted crowns, especially on 10mm implants, is a good idea. The forces on the molars are significantly higher than on front teeth so by splinting, your dentist is spreading out the chewing forces over the two implants reducing the risk of overloading any one implant.

I agree that splinted crowns, are better, stronger bite,
especially on front teeth, molars also but splinted in two crowns at a time. My dentist is fantastic and he decided on this and did it for me and it worked.

However one question, on my left upper jaw I have 6 splinted crowns and one of them was not fittet properly I can see the top screw of the implant there is a space between the implant and the crown. My dentist tried to take the crowns off,and make a new set, but it would not budge, so we decided to leave it alone in order not to jeoperdise the implants which are very well in the jaw. So I have to clean after every meal, using a water pick and or a brush pick. It is a bit annoying, but ok. Is there any other way to remove the crowns off implants safely? "Banging" them off does not work.

This depends on a couple of factors:
1. your bite
2. Proportion of the crown to the implant length
3. Type and size of implant placed
4. Your facial musculature and potential forces of chewing

Your surgeon and dentist need to assess these factors and make proper recommendations. Generally splinting the crowns adds cross support making each implant stronger, but again it should be based on the above parameters.